1. Field of the Invention
The present invention pertains to a method, apparatus and compositions for removing sludge and varnish precursors from a lubricating oil disposed within an internal combustion engine and for improving the oxidative stability of the lubricating oil. More particularly, the invention pertains to a method, apparatus and compositions for achieving this purpose by contacting the oil with an insoluble particle having a dispersant functional group and in some cases also an antioxidant functional group. The particle may be in the form of a porous slab, a thin film, or in the form of discrete particles which are within a circulating oil system but do not have a core substrate. These discrete particles may be "encaged", i.e. held inside of some large structural member by means of filter paper, wire mesh or by some other suitable means.
2. Description of Related Art
It is known in the art that during the combustion of fossil fuels, for example, gasoline or diesel fuel, in an internal combustion engine, polar hydrocarbon contaminants are formed due to incomplete combustion of the fuel. Typical contaminants include low molecular weight polar alkyl compounds such as alcohols, aldehydes, ketones, carboxylic acids, and the like. These contaminants are sludge and varnish precursors which pass into the lubricating oil with combustion blow-by gases where they contact water in the oil and agglomerate to form an emulsion which is commonly referred to as sludge. Sludge and varnish precursors can also arise from oil oxidation. The presence of sludge in the oil is undesirable because it tends to increase oil viscosity, promote the presence of varnish on hot engine parts, and plug oil passageways. The most common solution in the art for this problem has been to incorporate dispersants and antioxidants in the lubricating oil to increase the ability of the oil to suspend sludge. While this decreases the detrimental effect on viscosity, varnish, and passageway plugging, over time the ability of an oil to protect an engine becomes limited. A particular problem is that commonly used dispersants suspend the sludge in such a finely divided form that the sludge passes through oil filters and remains in the oil with subsequent viscosity increase rather than being removed by the filter. It would therefore be most desirable to employ a method for removing sludge and varnish precursors from a lubricating oil and thereby avoid the undesired result of leaving the sludge suspended in the oil.
It is known from U.S. Pat. No. 5,042,617, which is incorporated herein by reference, that compounds having dispersant functional groups (i.e. a functional group that complexes or reacts with sludge and varnish precursors) can be used within the oil circulation system of an internal combustion engine when such compounds are incorporated on a substrate. The present invention greatly improves on this method since the need for a substrate is eliminated. This substantially saves on the space required in an oil filter and significantly increases the amount of space available to accommodate removed sludge and varnish precursors. Elimination of the substrate also represents a cost savings. Retaining the particles of composition having a dispersant functional group or an antioxidant functional group on or between sheets of filter paper in order to keep them from moving about is very different from intimately depositing these compounds on a substrate.
It has now been found that the presence of sludge can be significantly decreased in circulating lubricating oils by contacting the sludge and varnish precursors with discrete particles of a composition having a dispersant functional group with or without an antioxidant functional group that is encaged within the circulating oil system, but not intimately adhered to or immobilized on a substrate. It is believed that the sludge and varnish precursors complex with the dispersant functional group and become immobilized on the particles. Preferably, the dispersant functional group is a crosslinked polyethylene amine which is in the form of discrete particles encaged within a conventional oil filter.